A coker unit is an oil refinery processing unit that converts residual oil from a separate oil refinery processing unit, such as a vacuum distillation column or an atmospheric distillation column, into low molecular weight hydrocarbon gases, naphtha, light and heavy gas oils, and petroleum coke. A coker unit uses heat to thermally crack long chain hydrocarbon molecules in is the residual oil feed into shorter chain molecules. The byproduct of this process is petroleum coke (or “coke”).
One type of coker unit is a delayed coker. A delayed coker is typically comprised of a main fractionator, a heat source, and at least one pair of coke drums. Residual oil feed from a separate refinery processing unit is pumped into the bottom of the main fractionator, which essentially is a distillation column. From there it is pumped, along with some injected steam to a furnace and heated to its thermal cracking temperature. It is then transported to one of the two coke drums where the majority of thermal cracking takes place. Lighter components are generated in vapor phase and directed back to the main fractionator where it is separated into various boiling point fractions. Coke remains in the coke drum. Once the first coke drum is filled with solid coke, the residual oil feed is directed from the furnace to the second coke drum. While the second coke drum is filling, coke from in the first coke drum is removed. Removal of the coke typically consists of quenching the coke with water to cool it followed by removing the top and bottom heads of the coke drum. Solid coke is then cut from the coke drum with a high pressure water jet. It then falls into a designated area for reclamation or storage.
Delayed coking is an important but slow process. One cycle generally takes more than ten hours to complete. This can create a bottleneck for other refinery processes. Reducing the delayed coker cycle time will increase throughput and in turn increase efficiency of the delayed coker and the refinery as a whole.
Delayed coking produces different types of cokes—i.e. coke with different morphology. Selected aromatic feedstocks may be used in the delayed coking process to produce high-quality needle cokes for graphite production. More commonly, though, more porous coke is produced. This more porous coke comes in predominantly two morphologies: (1) sponge coke and (2) shot coke. Generally, shot coke is preferred to sponge coke because it is more easily removed from the coke drum. Because it is more easily removed from the coke drum, the formation of shot coke can reduce delayed coking cycle times and ultimately improve refinery efficiency. Additionally, sponge coke can generally lead to “hot spots” within the coke drum which causes safety concerns during the coke removal process.
The mechanisms that produce sponge or shot coke are still not well understood in the industry. One technique involves measuring the ratio of the microcarbon residue (hereinafter “MCR”) to the quantity of asphaltenes present in the resid. If this ratio is less than 2, the resid will produce primarily shot coke. If the ratio is higher than 2, the resid will produce primarily sponge coke. The MCR to asphaltene ratio is based upon historical plant experience, but this ratio is only accurate at the extremes of the morphology spectrum and not at the intermediate values that are typically found in refineries.
A second approach found in the literature is to experimentally measure the quantity of aromatic carbon and heteroatoms (O, S, N) present in a precipitated asphaltene sample. If the ratio is greater than 11 then primarily sponge coke will be formed and if the ratio is less than 7 primarily shot coke will be formed. The region between 7 and 11 is a transitional region where the coke can create hot spots. This technique does not quantify the intermediate resids processed at some refineries, and the analysis is difficult to perform in a timely manner without specialized equipment. Furthermore, each of these techniques add an additional error to the measurement because the resid suspension is broken to remove the asphaltene molecules for analysis.
U.S. Publication No. 2015/0329784 to Siskin et al. takes a chemical approach to controlling coke morphology. The process comprises, in summary, mixing asphaltene derived from a shot coke-forming petroleum residual feed with or into a heated sponge coke forming petroleum residual feed to form shot coke directing asphaltene aggregates in the resid, holding the mixture of resid and the asphaltenes aggregates at an elevated temperature to allow co-aggregates of sponge coke and shot coke asphaltenes to form, and heating the heated resid containing the co-aggregates to a delayed coking temperature to form shot coke and thermally cracked coker products.
Other articles in the technical literature by Siskin and Kelemen together with their colleagues have provided insights into the possibilities of controlling coke morphology. See, for example. Siskin et al, “Asphaltene Molecular Structure and Chemical Influences on the Morphology of Coke Produced in Delayed Coking”, Energy & Fuels 2006, 20, 1227-1234; Siskin et al, “Chemical Approach to Control Morphology of Coke Produced in Delayed Coking,” Energy & Fuels, 2006, 20, 2117-2124; Kelemen et al, “Delayed Coker Coke Morphology Fundamentals: MeChanistic Implications Based on XPS Analysis of the Composition of Vanadium and Nickel-Containing Additives During Coke Formation,” Energy & Fuels 2007, 21, 927-940. In addition, a series of patents and applications from ExxonMobil Research and Engineering Company presented different proposals for promoting the production of a free-flowing shot coke during the delayed coking process; publications of these include U.S. Pat. Nos. 7,374,665; 7,871,510; WO 03/048271; WO 2007/050350; WO 2004/104139; WO 2005/113711; WO 2005/113712; WO 2005/113710; WO 2005/113709; WO 2005/113709; WO 2005/113708; WO 2007/058750.
Still others have observed changes in coke morphology with various operating parameters of the delayed coker—e.g. feed rate, pressure, and temperature, See, e.g. Michael Volk et al., Fundamentals of Delayed Coking Joint Industry Project, Univ. of Tulsa (2005). Other parameters that have been found to have an effect on coke morphology are recycle ratio and providing hydrogen donating additives to the feedstock, See, e.g., Aijun Guo et al., “Investigation on shot-coke-forming propensity and controlling of coke morphology during heavy oil coking,” 104 Fuel Processing Tech. 332 (2012).
While each of these publication describe factors that one may use to effect coke morphology, the number of variables present in the real world make such methods impractical. To truly ensure that the delayed coking process forms the preferred coke morphology, there is a need to monitor the coke morphology in situ while the coke is being made. Acoustic methods have been studied which show that attenuation and sonic speed can be correlated with coke morphology. However, the attenuation in using such methods is too high for field application. The currently disclosed process measures the AC impedance of the coke between one or more pairs of electrodes and determines coke morphology (e.g. sponge coke vs. shot coke) by using a correlation between coke morphology and AC impedance.